In recent years, in photolithography, along with high integration and high functionality of integrated circuits, microsizing of integrated circuits has been progressing, and an exposure device is required to form an image of a circuit pattern on a wafer with a high resolution in a deep focal depth, whereby blue shift of the exposure light source is in progress. The exposure light source has been advanced from the conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) or KrF excimer laser (wavelength: 248 nm), and now an ArF excimer laser (wavelength: 193 nm) is being used. Further, in order to be prepared for an integrated circuit for the next generation where the line width of a circuit pattern will be less than 100 nm, it is considered to be prospective to employ a F2 laser (wavelength: 157 nm) as the exposure light source, but it is considered that even this can not cover beyond a generation of a line width of 70 nm.
Under these circumstances, a lithographic technique employing typically a light having a wavelength of 13 nm among EUV light (extreme ultraviolet light) as the exposure light source, has attracted attention, as it may be applied to the printing of feature of 50 nm or smaller. The image-forming principle of the EUV lithography (hereinafter referred to as “EUVL”) is the same as the conventional photolithography to such an extent that a mask pattern is transferred by means of an optical projection system. However, in the energy region of EUV light, there is no material to let the light pass therethrough. Accordingly, a refraction optical system can not be used, and an optical system will be required to be a reflection optical system in all cases.
The optical material for the exposure device to be used for EUVL is basically constituted of (1) a base material, (2) a reflective multilayer formed on the base material and (3) an absorber layer formed on the reflective multilayer. The optical material for the exposure device to be used for EUVL is a reflection type, and thus the base material is not necessarily required to have light transparency. However, an extremely low thermal expansion material having transparency has been desired so as to make evaluation or inspection possible for the purpose of evaluating homogeneity or surface smoothness by using e.g. an interferometer so that the base material will not deform even when irradiated with EUV light, or for the purpose of judging presence or absence of internal defects such as bubbles or striae by microscopic or visual inspection.
Further, a transparent low thermal expansion material is widely used for materials which are strictly required to have low thermal expansion properties and transparency, such as a material for an optical component, a material for a large reflector substrate, a material for a ring laser gyroscope, a material for a precision component such as a standard for precision measurement and an electronic material.
The extremely low expansion material having transparency may be a silica glass containing TiO2 (hereinafter referred to as “TiO2—SiO2 glass”) represented by UEL#7972 (tradename) manufactured by Corning Incorporated and a vitrified crystallized glass represented by ZERODUR (tradename) manufactured by SCHOTT. U.S. Patent application publication No. 2002/157421 discloses a method which comprises forming a TiO2—SiO2 porous glass body, converting it to a glass body, and then obtaining a mask substrate therefrom.
TiO2—SiO2 glass is known to be an extremely low thermal expansion material having a coefficient of thermal expansion smaller than quartz glass, and the coefficient of thermal expansion can be controlled by the TiO2 content in the glass, whereby it is possible to obtain a zero expansion glass having a coefficient of thermal expansion being close to zero. Accordingly, TiO2—SiO2 glass is prospective as a material to be used for an optical material for the exposure device for EUVL. However, in TiO2—SiO2 glass, the temperature range wherein the coefficient of thermal expansion is substantially zero, is limited to about room temperature. Further, since it contains a large amount of OH groups, there are absorptions at several wavelengths, e.g. near 2,700 nm.
On the other hand, a crystallized glass comprises a crystalline phase exhibiting negative thermal expansion and a glass phase exhibiting positive thermal expansion, and it can be a zero expansion material having a coefficient of thermal expansion being close to zero, by controlling a heat step for crystallization. Further, the crystal grains are small, and the difference in refractive index between the crystalline phase and the glass phase as a matrix is small, and accordingly the crystalline glass is transparent. Accordingly, there is a possibility to obtain a material excellent in thermal expansion characteristics by contriving the composition of the base glass or a heat treatment step. However, the change in dimension along with the change in temperature exhibits hysteresis due to structural relaxation, such being problematic in absolute dimensional accuracy. Further, an optical material for the exposure device for EUVL is required to have an extremely smooth surface, such as a surface having a roughness Ra of at most 0.15 nm, but a smooth surface is hardly obtained due to influence of the crystal grains.
In a material for an optical component, a material for a precision component such as a standard for precision measurement, an electronic material, etc., and in an optical component for an exposure device for EUVL, the temperature range wherein the coefficient of thermal expansion is substantially zero is preferably broad, but in a conventional TiO2—SiO2 glass, the temperature range wherein the coefficient of thermal expansion is substantially zero is limited to about room temperature. Further, a conventional crystallized glass has a problem in absolute dimensional accuracy since the change in dimension along with the change in temperature exhibits hysteresis due to structural relaxation, and a crystallized glass having a smooth surface can hardly be obtained.